Catalysts for the oxidation of alpha,beta-unsaturated aldehydes to alpha,beta-unsaturated carboxylic acids and process for their preparation

ABSTRACT

Catalyst for the oxidation of an Alpha , Beta -unsaturated aldehyde to an Alpha , Beta -unsaturated carboxylic acid, said catalyst comprising (A) a mixture of oxides of the elements antimony, molybdenum, vanadium and tungsten and/or compounds of said elements and oxygen, and (B) at least one oxide or oxygen containing compound of the elements lead, silver, copper, tin, titanium or bismuth, said catalyst having an atomic ratio of antimony to molybdenum to vanadium to tungsten to Group (B) elements of about 1 - 60 : 12 : 0.5 - 25 : 0.1 - 12 : 0.1 - 12. Processes for the use and preparation of the catalysts are provided.

[451 Nov. 20, 1973 CATALYSTS FOR THE OXIDATION OF ALPHA, BETA-UNSATURATED ALDEHYDES TO ALPHA, BETA-UNSATURATED CARBOXYLIC ACIDS AND PROCESS FOR THEIR PREPARATION Inventors: Jorg Hensel; Theodor Lussling, both of Grossauheim; Ewald Noll, Grosskrotzenburg; Hans Schaefer, Grossauheim; Wolfgang Weigert, Offenbach, all of Germany Assignee: Deutsche Goldund Silber-Schneideanstalt vormals Roessler, Frankfurt am Main, Germany Filed: Oct. 12, 1971 Appl. No.: 188,462

Foreign Application Priority Data Oct. 13, 1970 Germany P 20 50 155.7

US. Cl 252/455 R, 252/456, 252/458, 252/467, 252/469, 260/530 N Int. Cl B0lj 11/06, B01j 11/20, BOlj 11/22 Field of Search 252/456, 464, 467, 252/455 R, 469; 260/530 N [56] References Cited UNITED STATES PATENTS 3,409,665 11/1968 Brown et al. 260/530 N 3,574,729 4/1971 Gasson 252/467 X 3,408,392 10/1968 Yamagishi et al. 260/530 N 3,567,773 3/1971 Yamaguchi et a1... 252/467 X 3,644,509 2/1972 Allen 252/464 X Primary Examiner-Daniel E. Wyman Assistant Examiner-W. J. Shine Att0rneyFrancis C. Browne [5 7 ABSTRACT Catalyst for the oxidation of an a,B-unsaturated aldehyde to an a,B-unsaturated carboxylic acid, said catalyst comprising (A) a mixture of oxides of the elements antimony, molybdenum, vanadium and tungsten and/or compounds of said elements and oxygen, and (B) at least one oxide or oxygen containing compound of the elements lead, silver, copper, tin, titanium or bismuth, said catalyst having an atomic ratio of antimony to molybdenum to vanadium to tungsten to Group (B) elements of about 1 60 12 0.5 25 0.1 12 z 0.1 12. Processes for the use and preparation of the catalysts are provided.

28 Claims, No Drawings CATALYSTS FOR THE OXIDATION OF ALPHA, BETA-UNSATURATED ALDEHYDES TO ALPHA, BETA-UNSATURATED CARBOXYLIC ACIDS AND PROCESS FOR THEIR PREPARATION This invention relates to catalysts for the oxidation of a,B-unsaturated aldehydes to corresponding a,,B-unsaturated carboxylic acids, and especially the oxidation of acrolein to acrylic acid, and methacrolein to methacrylic acid. This invention also relates to methods of preparing and using such catalysts.

Numerous processes have been known for the oxidation of a,B-unsaturated aldehydes. The processes differ by way of reaction conditions, and especially by use of various catalysts. Oxides of elements which occur in several oxidation stages are often used. Thus, oxides of vanadium, molybdenum, etc. are especially suitable. The latter in most instances are used on a carrier substance consisting of silicon dioxide or aluminum oxide.

Among the known processes and catalysts there are only a few suitable for industrial use where high conversions and high yields of product, such as acrylic acid in short contact times are required. Thus, in English patent 903,034, among others, molybdenum-antimony, molybdenum-tungsten and molybdenum-vanadium oxide catalysts are described. These catalysts can be used to convert acrolein to acrylic acid, but with relatively small yields. According to Belgian patent 698,273, a contact catalyst which contains molybdenum, vanadium and tungsten oxide gives somewhat more favorable results. It has turned out, however, that the yields of acrylic acid obtainable thereby will not permit economical production because the throughputs of acrolein possible on such catalysts are limited. For economical oxidation on an industrial scale, a catalyst is needed which remains active for a relatively long period of time making possible throughputs which are profitable in large scale industrial processes.

Accordingly, this invention provides a catalyst for the oxidation of an a,/3-unsaturated aldehyde to an a,B-unsaturated carboxylic acid, said catalyst comprising (A) a mixture of oxides of the elements antimony, molybdenum, vanadium and tungsten and/or compounds of said elements with oxygen, and (B) at least .one oxide or ox ygen containing compound of the elements lead, silver, copper, tin, titanium or bismuth, said catalyst having an atomic ratio of antimony to molybdenum to vanadium to tungsten to Group (B) elements of about 1 60 l2 0.5 25 0.l l2 0.l 12. The atomic ratio is preferably about 3-40212:l-l2:0.l-6:0.l-6. A particularly preferred atomic ratio is about 3-l5:l2:l-8:0.5-3:0-

This invention also provides a process for preparing a catalyst comprising (A) thoroughly mixing at least one compound selected from the group consisting of oxides, nitrates and ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium or tungstenwith at least one compound selected from the group consisting of oxides and nitrates of lead, silver, copper, tin, titanium or bismuth, (B) heating the resulting mixture in a first stage for about 2-300 minutes at about 150-275C., followed by (C) heating the mixture in a second stage in the presence of an oxygen containing gas for about 2-300 minutes at about 400-480C.

Further, this process provides a method for preparing the novel catalyst of this invention comprising (A) thoroughly mixing aqueous solutions or suspensions of at least one compound selected from the group consisting of oxides, nitrates and ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium or tungsten with at least one compound selected from the group consisting of oxides and nitrates of lead, silver, copper, tin, titanium or bismuth, (B) heating the resulting mixture at about 50l00C. while stirring and at a pH of about 2-7, (C) separating the resulting catalyst mass from the aqueous phase, (D) drying and heating the catalyst mass in a first stage for about 2-300 minutes at about 150-275C., followed by (E) heating the catalyst mass in a second stage in the presence of an oxygen containing gas for about 2-300 minutes at about 400480C.

Also, there is provided a process of using the novel catalysts of this invention which comprises oxidizing in a vapor phase an a,B-unsaturated aldehyde to an a,B-unsaturated carboxylic acid.

The catalysts of this invention can be used as such. Preferably, the catalysts are applied to or mixed with a carrier substance. Suitable carrier substances are for example, aluminum oxide and silicon dioxide. Others will be apparent to the art skilled. Preferably, the catalysts contain a mixture of carriers consisting of one or more components with a small specific surface (0.5 30 m /g), and one or more'components with a relatively large surface area (50 500 m /g). Particularly preferred as carrier materials are highly dispersed silicon dioxide or diatomite, mixtures of highly dispersed silicon dioxide and diatomite and/or montmorillonite.

The activity of the catalyst can be controlled by varying the weight ratio of the catalyst to the carrier, and especially by varying the ratio of carrier components with a large and small surface area.

The production of the catalysts can be accomplished by thoroughly mixing oxides and/or nitrates and/or ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium and tungsten with one or more of the oxides and/or nitrates of the elements lead, silver, copper, tin, titanium and bismuth, optionally with a carrier material. The resulting mixture is then heated in a first step for about 2 300 minutes, preferably about 10 150 minutes. Heating is conducted at about 150 275C., preferably about 200- 250C. Subsequently, the mixture is heated in a second stage for about 2 300 minutes, preferably about 2 60 minutes. About 5 30 minutes is particularly preferred. Heating in the second step is conducted at about 400 480C.,preferably about 420 460C., in the presence of oxygen containing gases, preferably air.

Preferably, the production of the catalysts is accomplished by thoroughly mixing aqueous solutions or suspensions, for example pastes, of the oxides and/or nitrates and/or ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium and tungsten with one or more of the oxides and/or nitrates of the elements lead, silver, copper, tin, titanium and bismuth, optionally with a carrier material. The resulting mixture is heated at a pH of about 2 7, preferably 3 5, while stirring to about 50 C. The catalyst mass is separated from the aqueous phase, and dried and heated in a first stage for about 2 300 minutes, preferably about 10 minutes, at about 150 275C., preferably about 200 250C. Subsequently, the mass is heated in a second stage for about 2 300 minutes at about 400 480C, preferably 420 460C., in the presence of an oxygen containing gas, preferably air.

It will be understood that within the scope of the method just described, compounds of the catalytically effective elements which decompose thermally in a similar manner can also be used. Furthermore, the addition of thermally decomposable ammonium salts to the aqueous solution or suspension can increase the effectiveness of the catalyst. Ammonium nitrate is preferred. Also, the antimony oxide when used can be pretreated with nitric acid.

According to a preferred embodiment of the invention, carrier materials consisting of one or more components with a low specific surface (0.5 30 mlg) together with carrier materials consisting of one or more components with large specific surfaces (50 500 m /g) can be used.

As carrier material, there can be used diatomite, highly dispersed silicon dioxide, or in a particularly preferred embodiment of the invention, mixtures of highly dispersed silicon dioxide and diatomite and/or montmorillonite. The montmorillonite can advantageously be heated to 900 l200C prior to mixing with silicon dioxide. Furthermore, the montmorillonite can be pretreated with acid prior to heating. Hydrochloric acid, nitric acid, phosphoric acid and sulfuric acid are preferred.

The catalyst can optionally be used in the shape of pressed articles, which preferably are molded from a mass of raw catalyst heated in a first step in an oxygen containing gas at about 150 275C, preferably about 200 250C. Prior to pressing, the mass can be ground if necessary and conventional molding agents, for example graphite, can be added in a conventional molding or extruding apparatus.

According to a particularly preferred embodiment of this invention, it is possible to achieve a graduation of the activity of the catalysts by treating the prepared catalyst with a slightly reducing or inert gas stream. Preferably, a nitrogen atmosphere heated to about 350 600C., preferably about 400 550C., is used.

The methods described will permit production of a catalyst having the desired activity and selectivity. The catalyst is particularly suitable for industrialapplication. Conventional fixed bed reactors consisting of pipe assemblies can be charged in a suitable manner with a catalyst graduated in activity as described above. Thus, the conversion of reactants can be made to occur evenly over the entire length of the reaction. Local temperature peaks which lead to side reactions, for example burning of the product to form carbon monoxide, carbon dioxide and water, will be avoided. The

graduation of the activity remains unchanged just as the activity and selectivity of the catalyst remain constant for a long time. The catalysts of thisinvention also can be used in fluidized bed reactors. The favorable characteristics of the catalysts of this invention originate, to a considerable extent, in a structure formed by the two stage heat treatment of the unburnt, dried mass of raw catalyst in the presence of oxygen containing gases, especially air.

The invention, is also useful for the oxidation of a,B-unsaturated aldehydes to a,B-unsaturated carboxylic acids in a vapor phase. Use of the catalysts for the oxidation of acrolein or methacrolein to acrylic acid or methacrylic acid in the vapor phase is preferred.

The catalysts of this invention differ from the catalysts used in known processes in that they not only make possible very high conversions but, surprisingly, they do not showany chang'e in their activity and selectivity even after several months of operating time. The catalysts of this invention have excellent selectivity for the oxidation of a,B-unsaturated aldehydes'to a,B-unsaturated carboxylicacids. For example, acrolein can be converted to acrylic acid with yields of more than percent. Finally, the catalysts of this invention make possible very high throughputs in industrial operations.

The oxidation of B-unsaturated aldehydes to a,B-unsaturated carboxylic acids with the use of catalysts according to this invention takes place in the customary manner by way of oxygen in the presence of water in the vapor phase. For the selection of the conversion conditions, there is a very broad latitude. The conversion is carried out mostly without use of pressure or with a slight excess pressure up to about 3 atm., and at temperatures of about 200 350C. The oxygen for the oxidation may originate from any given sources. Generally air is used. The quantity ratios among a,B-unsaturated aldehydes, air and water may vary within broad limits. Typical of the molar ratios between a,B-unsaturated aldehyde and air are l 2.5 l 30, preferably 1 2.5 l 10. Typical of the molar ratios between a,/3-unsaturated aldehyde and water are l 2 l 20, preferably 1 2 l 10. Preferably, feeds of 0.5 10, preferably 1.5 8 moles a,B-unsaturated aldehyde/l.hr are used.

This invention will be more clearly understood by reference to the following examples in which all parts, proportions and percentages are by weight unless otherwise indicated. Following are definitions of terms used herein.

Feed of M 1 a,Bunsaturated=*- 9 aldehyde bulk volume of 1 h catalyst timo mole converted a,B-

Conversion unsaturated aldehyde (17/) Tur mole used a,B-unsaturated' 0 V hyde 7,

Yield mo e product produced 100 EXAMPLE I 87.5 g antimony-(lll)-oxide (Sb O is pretreated by heating while stirring with 200 ml concentrated nitric acid until boiling. When no more nitrous gases escape,

the product is sucked off and washed with water. 212 g ammonium heptamolybdate [(NH,).M0,0,, 4H,O]

are dissolved in 3.5 liters water at below 50C. The pretreated antimony oxide, 212 g diatomite (Merck, DAB 6 BET surface 3.5 m /g), 24.2 g copper nitrate [Cu(NO 3 H O] dissolved in 50 ml water, 35.1 g ammonium monovanadate (NH,VO dissolved in 1 liter hot water and 30.4 g ammonium dodecatungstate [(NH W,,O suspended in 50 ml hot water, are added to the heptamolybdate solution while stirring. The mixture is brought to pH 3 with nitric acid, and is heated at boiling for two hours while stirring. Subsequently, the mixture is largely freed of water on a roller drier, is heated in air for 20 minutes to 250C., and is then cooled. After the addition of 3 percent by weight graphite powder, it is pressed into tablets of 5 mm diameter. The pressed articles subsequently are heated in a rotary kiln to 450 C. in air (dwell time in the rotary kiln about 8 minutes). The resulting catalyst contains antimony, molybdenum, vanadium, tungsten and copper in a molar ratio of6 12 3 1.2 l and diatomite as a carrier. The mechanical strength of the tablets expressed by the average breaking pressure in the case of radial pressure amounts to 1 kp.

65 ml of this catalyst are added to a solid bed reactor made of high grade steel with an inside diameter of 20 mm and 350 mm length. The reactor is heated by means of a salt bath to 272C. A gas mixture of acrolein, air and water vapor in a molar ratio of 1 l is passed across the catalyst at this temperature. The rate of addition of the acrolein is 2.65 mole/1h. An acrolein conversion of 99 percent and an acrylic acid yield of 91 percent based on the initial quantity is achieved.

EXAMPLE 2 In accordance with the procedure of Example 1, a catalyst is produced which has the same composition, but a carrier mixture of diatomite (BET surface 3.5 m /g) and highly dispersed silicon dioxide powder [Aerosil 200 (BET surface 170 m /g)] is used. The starting substances are 875 g antimony oxide, 2120 g ammonium heptamolybdate, 351 g ammonium monovanadate, 304 g ammonium paratungstate, 242 g copper nitrate, 1093 g highly dispersed silicon dioxide powder and 1093 g diatomite. The mechanical strength of the tablets expressed by the average breaking pressure in the case of radial pressure is 4 kp.

Use of this catalyst in the reactor of Example 1 at a salt bath temperature of 246C. results in an acrolein conversion of 98.7 percent and an acrylic acid yield of 86.0 percent based on the initial products. The special advantage of the catalyst lies in the fact that it permits use of a quite low operating temperature of 246C. with very good conversion and yield values.

EXAMPLE 3 According to the procedure of Example 2, a catalyst is produced with the difference that the pH value of the aqueous solution is not 3 but 1. i

The highest yield on the initial quantity of products of acrylic acid of this contact mass amounts in this case only to 59 percent at a salt bath temperature of 261C. and an acrolein turnover of 77.5 percent. Thus, the preferred embodiment of this invention, which involves mixing the starting substances in an aqueous environment at pH values between 2 and 7, has a favorable effect on the conversion and yield.

EXAMPLE 4 According to the procedure of Example 2, a catalyst is produced, with the difference that the diatomite is replaced by an equal quantity of montmorillonite (BET surface 5.4 m lg) which previously had been heated for 5 hours to 1000C. The mechanical strength of the catalyst tablets expressed by the average breaking pressure at radial pressure is 8 kp. As compared to the catalyst tablets obtained according to Example 1, this is an eight-fold increase in strength.

At a salt bath temperature of 268C, 93 percent of the acrolein used is converted and acrylic acid is formed at a yield of 82 percent.

EXAMPLES 5 .-a 10- The catalysts of Examples 5 10 are prepared just as in Example 1. Besides the same quantity in each case of antimony trioxide (87.5 g), ammonium heptamolybdate (212 g), ammonium monovanadate (35.1 g) and ammonium paratungstate (30.4 g), the following are used:

Example Addition Diatomite Aerosil Number 200 5 8gTi0 110.0g 110.0g 6 33.1 g Pb(NO 92.0 g 138.0 g 7 17.0 g AgNO, 88.0 g 131.5 g 8 30.5 g l3i(0H),NO 126.2 g 126.2 g 9 15.1gSn0, 112.0g 112.0g

18.3 g Cu (NO ),.3H,O 10 24.9 g Pb(N0 188.0 g 47.2 g

Following the procedure outlined in Example 1, the following results are obtained:

Ex. Reaction Acrolein Acrylic Acid Yield No. Temperature Conversion on lnitial Products 5 262 C 96.4% 86.0% 6 265 C 97.0% 85.9% 7 262 C 97.5% 85.3% 8 270 C 96.4% 83.4% 9 258 C 94.7% 83.5% 10 284 C 94.5% 83.5%

EXAMPLES ll 13 The catalysts of Examples 1 1 13 are prepared using the procedure of Example 1. Besides the same quantity of antimony trioxide (87.5 g), ammonium monovanadate (35.1 g), ammonium paratungstate (30.4 g) and copper nitrate (24.2 g), the following are used:

Ex. Ammonium Highly Dispersed No. Heptamolybdate SiO, Diatomite 11 247.0g 114.0g 114.0g 12 282.5 g 123.5 g 123.5 g 13 177.0 g 97.0 g 97.0 g

The catalysts have the following composition:

Ex. 11 Sb, Mo 2.51 LM m Ex. 12 sh M0,, v. w Ex. 13 S 12 u 1. m

The following optimum results are achieved using the reaction conditions of Example 1:

Ex Temperature of Acrolein Acrylic Acid Yield No the salt bath Conversion on lnitial Quantity EXAMPLE 14 EXAMPLE 15 A catalyst is prepared according to the procedure of Example 2, with the difference that 400 g ammonium nitrate are added to the aqueous solution.

Standard test conditions at 246C. results in an acrolein conversion of 97.6 percent and a yield of 86.3 percent acrylic acid based on the initial quantities.

What is claimed is:

1. Catalyst for the oxidation of an a,B-unsaturated aldehyde to an afiunsaturated carboxylic acid, said catalyst consisting of (A) a mixture of oxides of the elements antimony, molybdenum, vanadium and tungsten, and (B) at least one oxide of the elements lead, silver, copper, tin, titanium or bismuth, said catalyst having atomic ratio of antimony to molybdenum to vanadium to tungsten to Group (B) elements of about l-60:l2:0.5-25:0.l-l2:0.1-12.

2. Catalyst of claim 1 wherein the atomic ratio is about3-40: l2:1-12:0.16:0.1-6.

3. Catalyst of claim 1 wherein the atomic ratio is about3-l5:12:l-8:0.5-3:0.5- 3.

4. Catalyst of claim 1 containing at least one'carrier.

5. Catalyst of claim 3 containing at least one carrier having a specific surface area of about 0.5 30 mlg and at least one carrier having a specific surface area of about 50 500 m lg.

6. Catalyst of claim 5 wherein at least one carrier is highly dispersed silicon dioxide or diatomite.

7. Catalyst of claim 6 wherein one carrier is montmorillonite.

8. Process for preparing the catalyst of claim 1 comprising (A) thoroughly mixing at least one compound selected from the group consisting of oxides, nitrates and ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium and tungsten with at least one compound selected from the group consisting of oxides and nitrates of lead, silver, copper, tin, titanium and bismuth, (B) heating the resulting mixture in a first stage for about 2-300 minutes at about l50275C., followed by (C) heating in a second stage the mixture in the presence of an oxygen containing gas for about 2-300 minutes at about 400-480C.

9. Process of claim 8 in which first stage heating is at about 200250C. for about -150 minutes.

' 10. Process of claim 9 in which second stage heating is about 420-460C. for about 2-60 minutes.

1 1. Process of claim 10 in which second stage heating is for about 5-30 minutes.

12. Process of claim 8 in which the mixture also contains at least one carrier substance, and the gas is air.

13. Process for preparing the catalyst of claim 1 comprising (A) thoroughly mixing aqueous solutions or suspensions of at least one compound selected from the group consisting of oxides, nitrates and ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium and tungsten with at least one compound selected from the group consisting of oxides and nitrates of lead, silver, copper, tin, titanium and bismuth, (B), heating the resulting mixture at about 50-100C. while stirring and at a pH of about 2-7,(C) separating the resulting catalyst mass from the aqueous phase, (D) drying and heating the catalyst mass in a first stage for about 2-300 minutes at about l50-275C., followed by (E) heating in a second stage the catalyst mass in the presence of an oxygen containing gas for about 2-300 minutes at about 400-480C.

14. Process of claim 13 in which the pH is about 3-5.

15. Process of claim 14 in which first stage heating is at about 200250C. for about l0-l50'minutes.

16. Process of claim 15 in which second stage heating is at about 420-460C. for about 2-60 minutes.

17. Process of claim 16 in which second stage heating is for about 5-30 minutes.

18. Process of claim 13 in which the mixture also contains a carrier substance, and the gas is air.

19. Process of claim '13 in which at least one thermally decomposable ammonium salt is added to the aqueous solution or suspension.

20. Process of claim 19 in which the thermally decomposible ammonium salt is ammonium nitrate.

21. Process of claim 13 using antimony oxide pretreated with nitric acid.

22. Process of claim 17 in which the carrier substance is at least one carrier having a specific surface area of about 0.5-30 mlg and at least one other carrier having a specific surface area of about 500-500 mlg.

23. Process of claim 22 wherein at least one carrier is highly dispersed silicon dioxide or diatomite.

24. Process of claim 22 wherein one carrier is silicon dioxide mixed with montomorillonite which is heated to about 900-l200C. prior to mixing with the silicon dioxide.

25. Process of claim 24 in which the monitmorillonite is pretreated with hydrochloric acid, nitric acid, phosphoric acid, or sulfuric acid.

26. Process of claim 13 in which the catalyst mass is mixed with a pressing agent and pressed into a pressed article prior to step (E).

27. Process of claim 26 in which the catalyst is heated .in a slightly reducing or inert stream of gas at about ing or inert stream of gas is nitrogen, and the pressingagent is graphite.

* l t l 

2. Catalyst of claim 1 wherein the atomic ratio is about 3 - 40 : 12 : 1 - 12 : 0.1 - 6 : 0.1 -
 6. 3. Catalyst of claim 1 wherein the atomic ratio is about 3 - 15 : 12 : 1 - 8 : 0.5 - 3 : 0.5 -
 3. 4. Catalyst of claim 1 containing at least one carriEr.
 5. Catalyst of claim 3 containing at least one carrier having a specific surface area of about 0.5 - 30 m2/g and at least one carrier having a specific surface area of about 50 - 500 m2/g.
 6. Catalyst of claim 5 wherein at least one carrier is highly dispersed silicon dioxide or diatomite.
 7. Catalyst of claim 6 wherein one carrier is montmorillonite.
 8. Process for preparing the catalyst of claim 1 comprising (A) thoroughly mixing at least one compound selected from the group consisting of oxides, nitrates and ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium and tungsten with at least one compound selected from the group consisting of oxides and nitrates of lead, silver, copper, tin, titanium and bismuth, (B) heating the resulting mixture in a first stage for about 2-300 minutes at about 150*-275*C., followed by (C) heating in a second stage the mixture in the presence of an oxygen containing gas for about 2-300 minutes at about 400*-480*C.
 9. Process of claim 8 in which first stage heating is at about 200*-250*C. for about 10-150 minutes.
 10. Process of claim 9 in which second stage heating is about 420*-460*C. for about 2-60 minutes.
 11. Process of claim 10 in which second stage heating is for about 5-30 minutes.
 12. Process of claim 8 in which the mixture also contains at least one carrier substance, and the gas is air.
 13. Process for preparing the catalyst of claim 1 comprising (A) thoroughly mixing aqueous solutions or suspensions of at least one compound selected from the group consisting of oxides, nitrates and ammonium salts of oxy acids of the elements antimony, molybdenum, vanadium and tungsten with at least one compound selected from the group consisting of oxides and nitrates of lead, silver, copper, tin, titanium and bismuth, (B), heating the resulting mixture at about 50*-100*C. while stirring and at a pH of about 2-7, (C) separating the resulting catalyst mass from the aqueous phase, (D) drying and heating the catalyst mass in a first stage for about 2-300 minutes at about 150*-275*C., followed by (E) heating in a second stage the catalyst mass in the presence of an oxygen containing gas for about 2-300 minutes at about 400*-480*C.
 14. Process of claim 13 in which the pH is about 3-5.
 15. Process of claim 14 in which first stage heating is at about 200*-250*C. for about 10-150 minutes.
 16. Process of claim 15 in which second stage heating is at about 420*-460*C. for about 2-60 minutes.
 17. Process of claim 16 in which second stage heating is for about 5-30 minutes.
 18. Process of claim 13 in which the mixture also contains a carrier substance, and the gas is air.
 19. Process of claim 13 in which at least one thermally decomposable ammonium salt is added to the aqueous solution or suspension.
 20. Process of claim 19 in which the thermally decomposible ammonium salt is ammonium nitrate.
 21. Process of claim 13 using antimony oxide pretreated with nitric acid.
 22. Process of claim 17 in which the carrier substance is at least one carrier having a specific surface area of about 0.5-30 m2/g and at least one other carrier having a specific surface area of about 500-500 m2/g.
 23. Process of claim 22 wherein at least one carrier is highly dispersed silicon dioxide or diatomite.
 24. Process of claim 22 wherein one carrier is silicon dioxide mixed with montomorillonite which is heated to about 900*-1200*C. prior to mixing with the silicon dioxide.
 25. Process of claim 24 in which the monitmorillonite is pretreated with hydrochloric acid, nitric acid, phosphoric acid, or sulfuric acid.
 26. Process of claim 13 in which the Catalyst mass is mixed with a pressing agent and pressed into a pressed article prior to step (E).
 27. Process of claim 26 in which the catalyst is heated in a slightly reducing or inert stream of gas at about 350*-600*C. following step (E).
 28. Process of claim 27 in which the catalyst is heated at about 400*-550*C. after step (E), the slightly reducing or inert stream of gas is nitrogen, and the pressing agent is graphite. 